FMCW Radar Concepts Challenges Implementation Results A joint project Thanks to BSU Department of Geosciences Hans-Peter Marshall 1

FMCW Outline Some radar history and evolution FMCW concepts and benefits Design Considerations Testing and Results Refinements – Current and Future 2

Early History Thanks, Wikipedia et al 1865ScotlandJames Clerk Maxwell -- Theory of the Electromagnetic Field 1886GermanyHeinrich Hertz demonstrated RF reflections 1897ItalyGuglielmo Marconi demonstrated long distance transmission of electromagnet waves using a tent pole – l’antenna 1904GermanyChristian Hülsmeyer – telemobiloscope for traffic monitoring on water in poor visibility. First radar test 1917U.S.Nikola Tesla outlined radar concept 1921U.S.Albert Wallace Hull invented the Magnetron – efficient transmitting tube 1936U.S.Hetcalf & Hahn, GE, develop the Klystron 1939UKRandall & Boot build small powerful radar with multicavity magnetron, installed on B-17, and could see German submarines at night and in fog Many countries Radar technology development mushrooms during the war years in USA, Russia, Germany, France, Japan 3

Some Historical Perspective 4

More about Pulse Chirp Radar Increase BW with wider pulse by sweeping the frequency during the pulse Reconstruct narrow pulse with dispersive delay line in the receiver (Pulse compression). Practical approach using ASP methods when DSP capabilities were slow and expensive. 5

Using a SAW Filter as a Chirp ASP 6

Evolution from Chirp Radar to FMCW FMCW is the logical extension to Chirp Radar when: – DSP capabilities become practical. – PLL technology evolved to support highly linear frequency sweeping 7

What were the most important commercial outcomes from WWII Radar Research? The Microwave Oven which caused the FCC to abandon the 2.45 GHz band as licensable frequencies which enabled WiFi and Bluetooth bands 8

FMCW Outline Some radar history and evolution FMCW concepts and benefits Design Considerations Testing and Results Refinements – Current and Future 9

General FMCW Benefits Relative to Pulse Radar +Constant power improves transmitter efficiency +Ability to choose frequency ranges of operation +Lower cost to achieve wider bandwidth +More constant power over bandwidth of operation +More difficult to detect and jam -Requires lots of DSP data analysis -Requires very linear FM swept signal 10

FMCW Sawtooth Wave Concept Carrier Frequency f C Time f IF stst srsr δfδf δtδt T FFT Resolution: Splitter s IF stst srsr d LPF BW 11

FMCW Triangle Wave Concept Time f IF stst srsr δfδf δtδt T FFT Resolution: Carrier Frequency f C 12

FMCW Triangle with Doppler Shift Time τ stst srsr f IFu f IFd fdfd Carrier Frequency f C δfδf δtδt 13

The Radar Power Equation (1) Consider the power density arriving at the target from the transmitter: where is the transmitted power is the transmitter antenna gain is the distance from transmitter to target 14

The Radar Power Equation (2) The power reflected from the target toward the radar is: where σ is the Radar Cross Section (RCS). The power density received at the radar antenna is: 15

The Radar Power Equation (3) The received power from the receive antenna is: where A r is the effective area of the receive antenna. Combining the power equations results in the received power being: 16

Summary – All you need to know about FMCW Radar 17

FMCW Outline Some radar history and evolution FMCW concepts and benefits Design Considerations Testing and Results Refinements – Current and Future 18

The Objective Develop a radar system that can, from a distance: Profile the bottom surface of a saline ice sheet and Determine if there is oil under the sheet 19

System Constraints Minimum frequency for antennas is 500 MHz Maximum frequency for saline ice penetration is 2 GHz Therefore, maximum BW is 1.5GHz 20

System Implications Signal source – Heterodyne vs. YIG – Spurious signals – Nonlinearity – Phase noise IF frequency response Digitization resolution I/Q Demodulator 21

Signal Source The frequency range is 0.5 – 2.0 GHz Multi-octave sources in this range are either: – YIG Oscillator Terrific multi-octave bandwidth capabilities Have low Q and performance problems below 1-2 GHz Specified “nonlinearity” is typically 1% -- differential or integral? Expensive, requires a lot of power – Heterodyne VCO oscillators VCO oscillators are generally limited to a single octave of frequency range. Multi-octave source can be created by heterodyning two VCOs. Heterodyne oscillators create spurious signals. Slope of frequency vs. voltage is typically 2:1 or more Spurious signals create images at multiples of the distance of a dominant reflection. 22

Heterodyne Signal Source stst VCO 2 VCO 1 f2f2 f1f1 vt 2 vt 1 2 nd |f 2 - f 1 | 2 nd f 2 + f – 7.1 GHz 5.9 – 5.1 GHz 0.4 – 2.0 GHz 12.2 – 12.2 GHz 2.2 GHz LO RF Frequencies are selected to keep the third order product |2f 1 - f 2 | out of the range of interest. 23

System Implications Signal source – Heterodyne vs. YIG – Spurious signals – Nonlinearity – Phase noise IF response Digitization resolution I/Q Demodulator 24

Heterodyne Signal Source and associated spurious signals stst VCO 2 VCO 1 f2f2 f1f1 vt 2 vt 1 2 nd |f 2 - f 1 | 2 nd f 2 + f – 7.1 GHz 5.9 – 5.1 GHz 0.4 – 2.0 GHz 12.2 – 12.2 GHz 2.2 GHz LO RF 4 th |2f 2 - 2f 1 | 6 th |3f 2 - 3f 1 | 5 th |2f 2 - 3f 1 | Frequencies are selected to keep the third order product |2f 1 - f 2 | out of the range of interest. 0.8 – 4.0 GHz 1.1 – 5.1 GHz 1.2 – 6.0 GHz 3 rd |2f 1 - f 2 |3.1 – 5.5 GHz 25

Spur Table m n LORF

Classic Spur Chart 27

System Implications Signal source – Heterodyne vs. YIG – Spurious signals – Nonlinearity – Phase noise IF response Digitization resolution I/Q Demodulator 28

Nonlinearity then 29

Nonlinearity Requirements 30

System Implications Signal source – Heterodyne vs. YIG – Nonlinearity – Spurious signals – Phase noise IF response Digitization resolution I/Q Demodulator 31

Phase Noise 32

Phase Noise Cancellation 33

Phase-Locked Loop Solution to nonlinearity and phase noise VCO 2 f LO f RF vt 2 vt 1 stst 2.2 GHz Compensator Shaping Ckt PFD f REF 34 N.M Charge Pump Divides loop gain by N.M f = 0 Zero below gain crossover Compensate for N.M, Tuning nonlinearities

AD4158 PLL IC 35

AD4158 Registers 36

Assembly Challenge 37

Loop Design Sufficient loop gain to achieve good linearity. Gain crossover to optimize phase noise. Requires reasonably constant loop gain. Frequency dividers play significant role in the loop gain – must be compensated for in shaping circuit. 38

Typical VCO Phase Noise 39

Leeson's oscillator noise model 40 D. B. Leeson, “A Simple Model of Feedback Oscillator Noise Spectrum,” Proceedings of the IEEE, February 1966, pp. 329 – 330. F = noise factor

80 MHz Crystal Oscillator Phase Noise 41 Source: Crystek

Phase Noise Contributions 42

Simulator Results 43 Plot from Analog Devices ADIsimPLL

Loop Gain 44 Plot from Analog Devices ADIsimPLL

Tuning Sensistivity 45

Shaping network 46

Shaping Results 47

System Implications Signal source – Heterodyne vs. YIG – Spurious signals – Nonlinearity – Phase noise IF response Digitization resolution I/Q Demodulator 48

IF Gain Shaping Radar Power Equation: Reflections from more distant objects are generally weaker than reflections from similar objects that are closer The IF frequency is proportional to distance, so a suitable frequency response can compensate for this Doing this compensation in the analog circuit provides considerable improvement in the A/D dynamic range. 49

IF Gain Response a system designed to have the IF response to be relatively independent of distance R would have the frequency response: 50

RF System Block Diagram 51

The Application With ground penetrating radar (GPR), the dominant signal is the reflection from the bottom surface. But it requires near-contact with the top surface Non-contact radar Dominant signal is from top surface. didi τ εrεr εrεr τbτb τtτt 52

Top and Bottom Reflections With Non-contact Radar 53 High attenuation in saline ice creates a huge difference in top surface and bottom surface reflected signal. Early data suggests that: The difference in the travel time of the two reflections: The difference in the f IF from the two reflections: For a 40 cm saline ice sheet with ε R = 4.5 and with α = 1.5 GHz/20 ms, the difference in f IF from the two reflections will be: The simple problem: Isolate and detect a signal that is 424 Hz away from another signal in the 500 MHz – 2 GHz range with more than a million times as much power!

Making Ice in Hanover, NH Testing the prototype 54

The Gantry Test 55

The Gantry 56

The Radar on the Gantry 57

Example of Gantry Result 58

Gantry Magnitude Example 59

An Anti-aliasing Filter is Necessary! 60

IF Gain Response a system designed to have the IF response to be relatively independent of distance R and to provide an n-pole anti-aliasing low-pass filter would have the frequency response: 61

Up/Down Testing 62

Up/Down Example 63

Zoom into the first up/down data 64

Developing an algorithm to track the surface reflection 65

Doing a mean of convolution to find reflections below the surface 66

And taking the derivative of the convolution 67 Bottom? Second Harmonic of Bottom?

System Implications Signal source – Heterodyne vs. YIG – Spurious signals – Nonlinearity – Phase noise IF response Digitization resolution I/Q Demodulator 68

The A/D Converter used 69 Consider the NI USB-6251 for 1.25 MS/s, 16-bit analog input; built-in connectivity; and more Two 12-bit analog outputs, 8 digital I/O lines, two 24-bit counters Use with the LabVIEW PDA Module for handheld data acquisition applications NIST-traceable calibration and more than 70 signal conditioning options Superior LabVIEW, LabWindows™/CVI, and Measurement Studio integration for VB and VS.NET Included NI-DAQmx driver software and additional measurement services

SNR A/D SNR (Ideal) = (6.02N ) dB. This noise power is distributed equally to all M/2 FFT bins, referred to as the FFT process gain. For a system with T = 20ms, f s = 100 kHz, M/2 = 1000, or 30 dB. With a 12-bit A/D, ideal SNR will be 104 dB. This assumes full-scale signal over entire sweep. 70

Example of A/D-related SNR From Analog Devices Tutorial MT

Typical Time Domain Signal 72 V RMS = 127 mV

More on SNR The number of usable bits is around 8.2 bits. This would lead to a SNR = 80 dB. 73

Fresh water ice results 74

Profile with shaping and LPF 75

Some Results 76